I see some people have recommended white leghorns.
I raise white leghorns to sale eggs and chicks. I can tell you they are great layers 6 or 7 eggs per hen summer and winter. They are very hardy birds. The one thing you will need to do for them make sure they get sprouts or leafy vegetables...
My main coop is 4'x4'x8' placed inside a 7'x14' run 7' high.
We have 14 hens and a roster in this one. (I could put up to 25 hens in there) It has 8 nesting boxes and a dirt floor. I put a 3' door on it . It make's easy to clean out and get the eggs.
Very cheap to build. Also check...
x2 on mtngirl35's post.
If you go to fast you will stress out your flock. The new chickens are seen as invaders and not new members of the flock. And you may even end up with dead birds and no egg production .